This invention relates to roller conveyors for transporting objects such as silicon wafers or memory disks in cassettes, as well as control systems for such roller conveyors.
Conveyors of various types have been developed, including belt conveyors, roller conveyors and conveyors with a combination of both belts and rollers which are driven by motors. Each type of conveyor serves a specific purpose and modifications are constantly being introduced to eliminate various deficiencies and also to meet specific requirements or needs. One of the drawbacks of prior art conveyors is that they are costly. Typical prior art belt or roller conveyors utilize complex control systems incorporating a complex and heavy wiring system lacking the intelligence of transferring objects such that collisions or chafting of objects may occur frequently, resulting in generation of contaminating particles. Thus, another of major drawbacks of prior art conveyors is their inability to maintain a clean environment free of small particles. In many industries, smoothness in movement of the objects and contamination control are key issues. For applications inside a cleanroom, generation of small particles due to slippage of a belt conveyor becomes a serious problem for maintaining a dust-free environment. For transporting delicate objects such as silicon wafers and memory disks, conveyors which start and stop abruptly can cause damage to the surfaces of the objects.